The invention has application to high accuracy gantry positioning systems having substantially perpendicular, serially stacked, motion stages. Such systems have application in die placement machines, wafer inspection machines and co-ordinate measuring machines, and to accurate positioning systems requiring free access to a substantially stationary working area.
References herein to gantry systems are to be understood as including multi-axis positioning systems whereby a device is required to be accurately positioned in a two dimensional plane or a three dimensional space. In a typical two dimensional arrangement, the device is supported by a carriage which is movable back and forth in a first direction along a gantry beam. The gantry beam is movable back and forth in a second direction which is typically perpendicular to the first direction. The gantry beam is typically supported at both ends by a pair of carriages. If movement in three dimensions is required, the device is movably supported on the carriage so that the device is moveable in a third direction which is typically perpendicular to both the first and second directions. The three directions are typically orthogonal XYZ axes.
Typical conventional positioning systems include gantries of a T or H-shaped configuration. In these positioning systems, measurement in the XY plane is performed by mounting machine-readable position-encoded scales in line with each of the X and Y actuators. The layout is simple to implement and the servo control architecture is straightforward.
FIG. 1 shows a plan view of such a system, being a gantry system with an H-shaped configuration. One application is an Surface Mount Technology (SMT) placement machine. The gantry system controls the positioning of devices 18 over a working area 19, for example the substrate surface of a SMT device.
The devices 18 are mounted on an X actuator carriage 14. The carriage is movable in an X direction along an X gantry beam 13. The ends of the X gantry beam are mounted on respective Y actuator carriages 11, 12 to provide for movement of the X beam in a Y direction. Respective sensors in the Y actuator carriages 11, 12 are used to determine the Y position of the X beam 13 and the X carriage 14 from respective machine-readable position-encoded scales 15, 16. A sensor in the X carriage 14 is used to determine the X position of the X carriage from a machine-readable position-encoded scale 17 which is fixed to the X gantry 13.
The disadvantage of such a system is the lack of information on parasitic errors which degrade the final overall positioning accuracy. Such errors occur in both axes and can include tilts and deviations from straightness. While parasitic errors can be compensated for to a certain extent, for example by an indirect calibration process such as volumetric mapping, the errors tend to change with time, for example due to long-term dimensional instability or thermal deformations in the gantry system or its components. The frequency of re-calibration has to be determined from the level of accuracy required, as well as from the stability of the environmental conditions. Even if re-calibration is performed periodically, non-correctable errors, for example random, non-repeatable errors in the mechanical system, will remain. These errors pose limits to the final level of positioning accuracy achievable with the system described above with reference to FIG. 1.
It is technically possible to freeze a two dimensional measurement grid with an absolute accuracy of micrometer level over an area of some 400 mm×400 mm. If desired, further improvements could be made by error mapping to achieve nanometer accuracies. Such a two dimensional grid can be made almost totally temperature independent by using zero expansion glass substrates. Examples of suitable materials include Zerodur or ultra low thermal expansion material (ULE), with a coefficient of thermal expansion (CTE) of 0±0.02 ppm.
Such a grid of an appropriate size can be employed at the mean working level for absolute error mapping of the gantry system. The process is fast and direct, and enables repeated measurement for improved accuracy within a reasonable timeframe in which thermal change has negligible effect on the dimensional stability of the mechatronic system.
FIG. 2 shows an end elevation of the gantry system of FIG. 1 with the addition of an appropriately sized grid 21 placed directly above the gantry system. The grid is placed with sufficient flatness and parallelism to the working area 19. A look-up sensor head 22 reads the instantaneous X and Y positions of the gantry. However, this reading if the XY position is made at the level of the grid 21 which is displaced in a vertical direction above the working area. As such, Abbe errors due to pitch and roll of the gantry beam and X carriage have to be kept to within reasonable levels, or compensated for.
The Abbe errors can be compensated for by making real-time pitch and roll measurements of the gantry relative to a flat reference surface which, conveniently, can be the grid 21 already provided for making XY measurements. With such a metrology system, no calibration is required and dimensional changes due to thermal effects can be readily measured and corrected, at least to some extent. In effect, the gantry no longer relies on the quality of its guiding system but rather on those of the measurement system. However, this system still suffers from the disadvantage of poor visibility of, and accessibility to, the working area 19 which is covered by the ‘overhead’ two dimensional grid 21.